Manual Section... (7) - page: signal

NAME

DESCRIPTION

Signal Dispositions

Each signal has a current
disposition,
which determines how the process behaves when it is delivered
the signal.

The entries in the "Action" column of the tables below specify
the default disposition for each signal, as follows:

Term

Default action is to terminate the process.

Ign

Default action is to ignore the signal.

Core

Default action is to terminate the process and dump core (see
core(5)).

Stop

Default action is to stop the process.

Cont

Default action is to continue the process if it is currently stopped.

A process can change the disposition of a signal using
sigaction(2)
or (less portably)
signal(2).
Using these system calls, a process can elect one of the
following behaviors to occur on delivery of the signal:
perform the default action; ignore the signal;
or catch the signal with a
signal handler,
a programmer-defined function that is automatically invoked
when the signal is delivered.
(By default, the signal handler is invoked on the
normal process stack.
It is possible to arrange that the signal handler
uses an alternate stack; see
sigaltstack(2)
for a discussion of how to do this and when it might be useful.)

The signal disposition is a per-process attribute:
in a multithreaded application, the disposition of a
particular signal is the same for all threads.

A child created via
fork(2)
inherits a copy of its parent's signal dispositions.
During an
execve(2),
the dispositions of handled signals are reset to the default;
the dispositions of ignored signals are left unchanged.

Sending a Signal

The following system calls and library functions allow
the caller to send a signal:

Temporarily changes the signal mask (see below) and suspends
execution until one of the unmasked signals is caught.

Synchronously Accepting a Signal

Rather than asynchronously catching a signal via a signal handler,
it is possible to synchronously accept the signal, that is,
to block execution until the signal is delivered,
at which point the kernel returns information about the
signal to the caller.
There are two general ways to do this:

*

sigwaitinfo(2),
sigtimedwait(2),
and
sigwait(3)
suspend execution until one of the signals in a specified
set is delivered.
Each of these calls returns information about the delivered signal.

*

signalfd(2)
returns a file descriptor that can be used to read information
about signals that are delivered to the caller.
Each
read(2)
from this file descriptor blocks until one of the signals
in the set specified in the
signalfd(2)
call is delivered to the caller.
The buffer returned by
read(2)
contains a structure describing the signal.

Signal Mask and Pending Signals

A signal may be
blocked,
which means that it will not be delivered until it is later unblocked.
Between the time when it is generated and when it is delivered
a signal is said to be
pending.

Each thread in a process has an independent
signal mask,
which indicates the set of signals that the thread is currently blocking.
A thread can manipulate its signal mask using
pthread_sigmask(3).
In a traditional single-threaded application,
sigprocmask(2)
can be used to manipulate the signal mask.

A child created via
fork(2)
inherits a copy of its parent's signal mask;
the signal mask is preserved across
execve(2).

A signal may be generated (and thus pending)
for a process as a whole (e.g., when sent using
kill(2))
or for a specific thread (e.g., certain signals,
such as
SIGSEGV
and
SIGFPE,
generated as a
consequence of executing a specific machine-language instruction
are thread directed, as are signals targeted at a specific thread using
pthread_kill(3)).
A process-directed signal may be delivered to any one of the
threads that does not currently have the signal blocked.
If more than one of the threads has the signal unblocked, then the
kernel chooses an arbitrary thread to which to deliver the signal.

A thread can obtain the set of signals that it currently has pending
using
sigpending(2).
This set will consist of the union of the set of pending
process-directed signals and the set of signals pending for
the calling thread.

A child created via
fork(2)
initially has an empty pending signal set;
the pending signal set is preserved across an
execve(2).

Standard Signals

Linux supports the standard signals listed below.
Several signal numbers
are architecture-dependent, as indicated in the "Value" column.
(Where three values are given, the first one is usually valid for
alpha and sparc,
the middle one for ix86, ia64, ppc, s390, arm and sh,
and the last one for mips.
A - denotes that a signal is absent on the corresponding architecture.)

The signals
SIGKILL
and
SIGSTOP
cannot be caught, blocked, or ignored.

Next the signals not in the POSIX.1-1990 standard but described in
SUSv2 and POSIX.1-2001.

Signal

Value

Action

Comment

SIGPOLL

Term

Pollable event (Sys V).

Synonym for SIGIO

SIGPROF

27,27,29

Term

Profiling timer expired

SIGSYS

12,-,12

Core

Bad argument to routine (SVr4)

SIGTRAP

5

Core

Trace/breakpoint trap

SIGURG

16,23,21

Ign

Urgent condition on socket (4.2BSD)

SIGVTALRM

26,26,28

Term

Virtual alarm clock (4.2BSD)

SIGXCPU

24,24,30

Core

CPU time limit exceeded (4.2BSD)

SIGXFSZ

25,25,31

Core

File size limit exceeded (4.2BSD)

Up to and including Linux 2.2, the default behavior for
SIGSYS, SIGXCPU, SIGXFSZ,
and (on architectures other than SPARC and MIPS)
SIGBUS
was to terminate the process (without a core dump).
(On some other Unix systems the default action for
SIGXCPU and SIGXFSZ
is to terminate the process without a core dump.)
Linux 2.4 conforms to the POSIX.1-2001 requirements for these signals,
terminating the process with a core dump.

Next various other signals.

Signal

Value

Action

Comment

SIGEMT

7,-,7

Term

SIGSTKFLT

-,16,-

Term

Stack fault on coprocessor (unused)

SIGIO

23,29,22

Term

I/O now possible (4.2BSD)

SIGCLD

-,-,18

Ign

A synonym for SIGCHLD

SIGPWR

29,30,19

Term

Power failure (System V)

SIGINFO

29,-,-

A synonym for SIGPWR

SIGLOST

-,-,-

Term

File lock lost

SIGWINCH

28,28,20

Ign

Window resize signal (4.3BSD, Sun)

SIGUNUSED

-,31,-

Term

Unused signal (will be SIGSYS)

(Signal 29 is
SIGINFO
/
SIGPWR
on an alpha but
SIGLOST
on a sparc.)

SIGEMT
is not specified in POSIX.1-2001, but nevertheless appears
on most other Unix systems,
where its default action is typically to terminate
the process with a core dump.

SIGPWR
(which is not specified in POSIX.1-2001) is typically ignored
by default on those other Unix systems where it appears.

SIGIO
(which is not specified in POSIX.1-2001) is ignored by default
on several other Unix systems.

Real-time Signals

Linux supports real-time signals as originally defined in the POSIX.1b
real-time extensions (and now included in POSIX.1-2001).
The range of supported real-time signals is defined by the macros
SIGRTMIN
and
SIGRTMAX.
POSIX.1-2001 requires that an implementation support at least
_POSIX_RTSIG_MAX
(8) real-time signals.

The Linux kernel supports a range of 32 different real-time
signals, numbered 33 to 64.
However, the glibc POSIX threads implementation internally uses
two (for NPTL) or three (for LinuxThreads) real-time signals
(see
pthreads(7)),
and adjusts the value of
SIGRTMIN
suitably (to 34 or 35).
Because the range of available real-time signals varies according
to the glibc threading implementation (and this variation can occur
at run time according to the available kernel and glibc),
and indeed the range of real-time signals varies across Unix systems,
programs should
never refer to real-time signals using hard-coded numbers,
but instead should always refer to real-time signals using the notation
SIGRTMIN+n,
and include suitable (run-time) checks that
SIGRTMIN+n
does not exceed
SIGRTMAX.

Unlike standard signals, real-time signals have no predefined meanings:
the entire set of real-time signals can be used for application-defined
purposes.
(Note, however, that the LinuxThreads implementation uses the first
three real-time signals.)

The default action for an unhandled real-time signal is to terminate the
receiving process.

Real-time signals are distinguished by the following:

1.

Multiple instances of real-time signals can be queued.
By contrast, if multiple instances of a standard signal are delivered
while that signal is currently blocked, then only one instance is queued.

2.

If the signal is sent using
sigqueue(2),
an accompanying value (either an integer or a pointer) can be sent
with the signal.
If the receiving process establishes a handler for this signal using the
SA_SIGINFO
flag to
sigaction(2)
then it can obtain this data via the
si_value
field of the
siginfo_t
structure passed as the second argument to the handler.
Furthermore, the
si_pid
and
si_uid
fields of this structure can be used to obtain the PID
and real user ID of the process sending the signal.

3.

Real-time signals are delivered in a guaranteed order.
Multiple real-time signals of the same type are delivered in the order
they were sent.
If different real-time signals are sent to a process, they are delivered
starting with the lowest-numbered signal.
(I.e., low-numbered signals have highest priority.)
By contrast, if multiple standard signals are pending for a process,
the order in which they are delivered is unspecified.

If both standard and real-time signals are pending for a process,
POSIX leaves it unspecified which is delivered first.
Linux, like many other implementations, gives priority
to standard signals in this case.

According to POSIX, an implementation should permit at least
_POSIX_SIGQUEUE_MAX
(32) real-time signals to be queued to
a process.
However, Linux does things differently.
In kernels up to and including 2.6.7, Linux imposes
a system-wide limit on the number of queued real-time signals
for all processes.
This limit can be viewed and (with privilege) changed via the
/proc/sys/kernel/rtsig-max
file.
A related file,
/proc/sys/kernel/rtsig-nr,
can be used to find out how many real-time signals are currently queued.
In Linux 2.6.8, these
/proc
interfaces were replaced by the
RLIMIT_SIGPENDING
resource limit, which specifies a per-user limit for queued
signals; see
setrlimit(2)
for further details.

Async-signal-safe functions

A signal handling routine established by
sigaction(2)
or
signal(2)
must be very careful, since processing elsewhere may be interrupted
at some arbitrary point in the execution of the program.
POSIX has the concept of "safe function".
If a signal interrupts the execution of an unsafe function, and
handler
calls an unsafe function, then the behavior of the program is undefined.

POSIX.1-2004 (also known as POSIX.1-2001 Technical Corrigendum 2)
requires an implementation to guarantee that the following
functions can be safely called inside a signal handler:

Interruption of System Calls and Library Functions by Signal Handlers

If a signal handler is invoked while a system call or library
function call is blocked, then either:

*

the call is automatically restarted after the signal handler returns; or

*

the call fails with the error
EINTR.

Which of these two behaviors occurs depends on the interface and
whether or not the signal handler was established using the
SA_RESTART
flag (see
sigaction(2)).
The details vary across Unix systems;
below, the details for Linux.

If a blocked call to one of the following interfaces is interrupted
by a signal handler, then the call will be automatically restarted
after the signal handler returns if the
SA_RESTART
flag was used; otherwise the call will fail with the error
EINTR:

*

read(2),
readv(2),
write(2),
writev(2),
and
ioctl(2)
calls on "slow" devices.
A "slow" device is one where the I/O call may block for an
indefinite time, for example, a terminal, pipe, or socket.
(A disk is not a slow device according to this definition.)
If an I/O call on a slow device has already transferred some
data by the time it is interrupted by a signal handler,
then the call will return a success status
(normally, the number of bytes transferred).

The following interfaces are never restarted after
being interrupted by a signal handler,
regardless of the use of
SA_RESTART;
they always fail with the error
EINTR
when interrupted by a signal handler:

The
sleep(3)
function is also never restarted if interrupted by a handler,
but gives a success return: the number of seconds remaining to sleep.

Interruption of System Calls and Library Functions by Stop Signals

On Linux, even in the absence of signal handlers,
certain blocking interfaces can fail with the error
EINTR
after the process is stopped by one of the stop signals
and then resumed via
SIGCONT.
This behavior is not sanctioned by POSIX.1, and doesn't occur
on other systems.